Use of a nesting platform by Gull-billed Terns and Black Skimmers at the Salton Sea, California

In 2006, we constructed an elevated nesting platform at the Salton Sea, California, and monitored its use by Gull-billed Terns and Black Skimmers over three subsequent breeding seasons. Black Skimmers were the first to colonize the platform with a total of five nests in 2006. In 2007 Gull-billed Terns colonized the platform with a total of 28 nests and the number of Black Skimmer nests increased to 20. Neither species nested on the platform in 2008. Low success for both species was probably influenced by at least two factors. First, when both species nested on the platform, nest densities were higher than is typical of their colonies on larger, earthen islands, and colony success may have been reduced by overcrowding. Second, lack of access to water may have reduced chicks’ ability to thermoregulate effectively in the hot environment of the Salton Sea. Refinements to the size, design, and location of artificial nesting habitats are necessary to enhance productivity of colonial groundnesting birds at the Salton Sea successfully. Artificial or constructed nesting habitats have often been employed to increase the number of suitable breeding sites for ground-nesting colonial birds such as terns (Lampman et al. 1996, Quinn and Sirdevan 1998, Spear et al 2007, Jenniges and Plettner 2008). Such artificial nesting habitats have been provided to (1) accommodate the growth of newly colonizing populations, (2) enhance reproductive success, (3) create novel sites that are relatively free of predators or that reduce competition for space with more aggressive species (such as gulls; Molina 2004), and (4) augment the number of alternative sites when the suitability of existing natural habitats declines with the encroachment of vegetation or when the protective isolation of islands diminishes as lake levels fall. Constructed habitats are typically located in lakes, impoundments, or other shallow and protected waters. These nesting habitats often take the more traditional form of sand, rock, and earthen islands (usually formed with bottom sediments). Floating platforms such as rafts (Dunlop et al. 1991, Lampman et al. 1996) also have been used when the deposition of dredged sediments is infeasible. When natural nesting sites are limited, rafts and barges have been used to promote the colonization of alternative nesting habitats when land-use conflicts arise and colony relocation is desired (Collis et al. 2002). In some cases breeding larids have spontaneously (i.e., without the aid of audio-visual attractants such as decoys or call broadcasters) colonized the surfaces of existing floating or fixed platforms such as moored barges (Molina pers. obs.), the surfaces of navigational buoys (Karwowski et al. 1995), or even the flat gravel roof tops of buildings (Fisk 1975, Coburn et al. 1997). 267 Western Birds 40:267–277, 2009 USE OF A NESTING PLATFORM BY TERNS AND SKIMMERS The Salton Sea is the largest terminal lake in California; this highly dynamic and productive saline ecosystem lies below sea level and lacks an outlet. Over the past decade, Gull-billed Terns (Gelochelidon nilotica) and Black Skimmers (Rynchops niger) breeding at the Salton Sea have experienced the loss of isolated nesting sites (islands) with declines in water level (Molina 2004). Since 2001, the continued decline of water levels has resulted in additional losses of the traditional nesting islands of Obsidian Butte, Morton Bay, and Elmore Desert Ranch by increasing mammalian predators’ access to these sites and further intensifying waterbirds’ demand for undisturbed nesting and resting sites (Molina 2004, 2007). There is no precedent in California for breeding Gull-billed Terns and Black Skimmers nesting on habitats other than earthor gravel-based islands (Molina 2008a, b) or rafts that mimic islands (Molina 2007). However, Gullbilled Terns (Molina and Erwin 2006), Least Terns (Sternula antillarum; Krogh and Schweitzer 1999), and Black Skimmers (Coburn et al. 1997) have spontaneously colonized elevated sites such as flat gravel roofs in coastal Louisiana and Florida. Given the propensity of larids in general to use artificial sites in other parts of North America, in 2006 we constructed a fixed and elevated platform at the south end of the Salton Sea to address the decline in the number of suitable colony sites and to enhance nesting for the Gull-billed Tern and its close nesting associate, the Black Skimmer. In this paper we report the number of breeding pairs, nest attempts, and young of Gull-billed Terns and Black Skimmers on this novel habitat and summarize the phenology of its use in 2006, 2007, and 2008. We also discuss the disadvantages and potential refinements of artificial nesting habitats at the Salton Sea.